Class: Conoidasida
Subclass: Coccidiasina
Order: Eucoccidiorida
Suborder: Eimeriorina

Isospora species are coccidians within the Apicomplexa, and are most closely related to the genus Eimeria. Both Eimeria and Isospora are highly host specific. The most significant difference between Eimeria and Isospora is that the latter may use a paratenic host in its life cycle, whereas the former does not.

Isospora suis is the most important coccidian of pigs around the world. Clinical disease associated with Isospora species is known as coccidiosis. Several species of Eimeria can also infect pigs, and although they have occasionally been associated with clinical disease in older pigs, compared to I. suis they are currently believed to be relatively unimportant.

The life cycle stages of Isospora species visible by standard microscopy include those associated with asexual (merogony) and sexual (gametogony and fertilization) reproduction within the enterocytes of the small intestine, the oocysts, and stages in the paratenic hosts, although these are rarely if ever sought for diagnosis. The intestinal stages – meronts, merozoites, macrogametocytes, microgametocytes, gamonts and oocysts, are typical for the genus, and can usually be seen in histological sections.

Isospora suis oocysts passed in feces of pigs are ellipsoidal to sub-spherical, measure up to approximately 20 by 15 µm and have a thin, smooth shell. Freshly passed oocysts contain one or two cells. Each sporulated oocyst of Isospora contains two sporocysts, each containing four sporozoites.

Isospora species undergo asexual (merogony) and then sexual (gametogony and fertilization) reproduction in the enterocytes of the small intestine. The infective stage is the sporulated oocyst which, following ingestion by a suitable host, hatches to release eight sporozoites. These sporozoites enter enterocytes and divide rapidly to form merozoites enclosed within a meront, which can come to occupy most of a host cell. Depending on the species of Isospora, a meront can contain up to several thousand merozoites. The infected cell then bursts, releasing the meronts into the intestinal lumen, from where they penetrate new enterocytes. Isospora suis has three generations of merogony in the enterocytes of the lower small intestine, and to a lesser extent the caecum and colon.

Eventually, merozoites entering host cells do not divide to produce meronts, but instead form microgametocytes ("male") and microgametocytes ("female") within the enterocytes. Each microgametocyte contains several microgametes, but each macrogametocyte contains only a single macrogamete. Next, the microgametocyte disintegrates, releasing the microgametes, which fertilize the macrogametocytes, forming gamonts which develop into unsporulated oocysts. Gametogony of I. suis occurs primarily in the lower small intestine. These oocysts burst from the enterocytes and are passed in feces. In the environment, over a few days under ideal conditions, the oocysts sporulate and are then infective. Infection of pigs is by ingestion of sporulated oocysts.

The pre-patent period for I. suis is approximately one to two weeks. There may be extra-intestinal stages of I. suis in pigs, but they are as yet unidentified and their significance is unknown.

If sporulated oocysts of Isospora species are ingested by a paratenic hosts, usually small mammals, they hatch and the sporozoites released penetrate into the intestinal wall and encyst intracellularly as single bradyzoites. The cysts can be found in a variety of tissues and organs, particularly mesenteric lymph nodes. The significance of paratenic hosts for I. suis has not been determined, but given the intensive management systems used for most pigs in Canada, it seems unlikely that they are important.

Life Cycle: Isospora suis

The most significant pathology caused by I. suis occurs in the small intestine and is associated mainly with gametogony. The major lesion is destruction of the mucosal epithelium.

Clinical disease associated with I. suis is seen most often in pigs seven to ten days old, and is characterised by recurrent diarrhea that does not respond to anti-microbials. Disease may be more frequent when and where environmental conditions are optimal for sporulation of the oocysts. A recent survey of 709 litters of piglets on 50 farms revealed a farm prevalence of 70% and 187 of the 709 litters infected.  In this study, infection with I suis significantly increased the likelihood of diarrhea compared to uninfected litters, while farms that washed farrowing crates with a detergent were significantly less likely to have I. suis in their pigs..

The history and clinical signs are helpful, as is the recovery of oocysts from feces by flotation. With I. suis, it is important to differentiate the oocyst from those of the apparently non-pathogenic Eimeria species of pigs. This is best done by examining sporulated oocysts.

In general, the more oocysts there are in feces, the more likely coccidia are to be the cause of clinical disease. With I. suis, there may be clinical signs suggestive of coccidiosis without oocysts in feces, and in this situation, repeat fecal samples should be examined and samples should be collected from several animals in the affected group. Some or all of the intestinal stages of the parasite can usually be seen histologically on post mortem.

Several products have been assessed for the treatment and control of Isospora suis, given to sows and/or piglets, but none has proved to be usefully effective.  There are no products approved in Canada for this parasite in pigs.